This program was initiated to elucidate a newly recognized modality of vaccination and to extend our long term interest in studying the immune response and clinical sequela of hepatitis C virus (HCV) infection. One of the advantages of genetic immunization is that the endogenously expressed proteins can be recognized by class I MHC molecules and expressed on the cell surface. The MHC-antigen complex on the cell surface can be recognized by cytotoxic T lymphocytes (CTL), which in turn are activated and attack infected cells. The possibility of inducing an immune response to HCV core protein using DNA immunization provides an attractive alternative to classic vaccination. There are many prob-lematic issues related to the vaccine development for hepatitis C. One of the major concerns is the genetic stability of the infectious agent, HCV. There are two hypervariable regions in the putative HCV envelope proteins. Immune escape mutants observed were attributed to have mutation in these regions. Experimentally infected chimpanzees and HCV patients were found to have repeated bouts of infection with either homologous or new strains of HCV. This could also be one of the reasons that more than 80 percent of the infection becomes chronic. Directly inducing strong cell-mediated immunity, especially protective cytotoxic T lymphocytes, may not only help in preventing initial HCV infection, but may even serve as immune modulation to overcome the infection. During the past year, we were able to evaluate the induction of antibodies to several different plasmid constructs containing both HCV structural and non-structural genes in mice. We were also able to develop the assay and to measure CTL activities in the mouse model.